The technology disclosed herein generally relates to communications satellite systems and methods, and more particularly relates to the broadcast of signals on a local-area basis with some of the signal frequency bands being repeated for different areas.
“Spot” broadcast beams, which are smaller than regional beams, have been used for satellite broadcasting. In accordance with one type of spot beam broadcast, a desired region, such as a country, is covered by a uniform grid of evenly spaced spot beams having equal sizes and output power levels. To assure complete area coverage, adjacent beam spots are overlapped. Different and non-overlapping frequency bands are assigned to the signals within each pair of adjacent beams to prevent cross-beam signal interference. In accordance with some configurations, four different frequency bands are employed, with each beam being separated from the next closest beam with the same frequency band by at least one other beam having a different frequency band. The uniform spot beams provide complete coverage of the desired larger regional area without significant interference between beams.
Uplink and downlink antennas may be implemented with any conventional antennas used in satellite communications. In various embodiments, antennas are implemented with digital or analog beamforming antennas having any number of independently-addressable transmit/receive elements. Examples of such antennas include various spot beam dishes, multi-beam feed antennas, direct radiating array antennas and/or phased array antennas which are commercially available.
This is great demand for communications satellites that have high throughput (i.e., (bit rate transmitted/bandwidth used)×bandwidth). This means that communications satellite antennas should have the capability to produce a large number of spot beams with advanced beamforming. Beamforming in the microwave frequency domain is prohibitive because the microwave beamforming components consume the satellite's power and mass resources. Performing onboard beamforming in the optical domain has the potential to ameliorate this resource problem. In addition, optical systems have more available spectral bandwidth
In accordance with currently proposed optical communications payload architectures, the optical beamforming is achieved utilizing active units. These units often include lasers, optical receivers and MEMS switching networks. These active units consume large amounts of the satellite's power and mass resources. Active units also greatly reduce the satellite's reliability.
A communications satellite system that uses passive rather than active optical components to process optical beams received from a ground station would be a welcome advance in the art.